PROJECT SUMMARY: Chronic low-back pain (CLBP) is a disabling condition with no available cure. About 30% of CLBP patients suffer from neuropathic back pain, which is thought to arise from an injury to the nerve root secondary to degenerated discs and/or local inflammation. Compared to CLBP without a neuropathic component, neuropathic CLBP is associated with more patient distress, especially in women, increased severity of medical co-morbidities and a 70% increase in health care cost. Despite extensive pre-clinical studies of peripheral nerve injury pain models and clinical research on neuropathic CLBP there is no effective analgesic treatment to date. This state of affair reflects our limited understanding of the pathophysiology of neuropathic CLBP and the need for novel targets for analgesic treatment. Accumulating evidence point to a critical role of the brain limbic and somatosensory circuitries in the pathophysiology of chronic pain in humans. Our pilot data is among the first to show smaller thalamus and altered thalamic and limbic system functional connectivity in neuropathic CLBP compared to non-neuropathic CLBP patients and healthy controls. Regardless, systematic mapping of these circuitries in neuropathic CLBP pain patients is still lacking. The objective of the study is to map structural and functional properties of the limbic and somatosensory circuitries in neuropathic CLBP in comparison to non- neuropathic CLBP patients and matched healthy controls. We hypothesize that neuropathic CLBP patients will show significant structural and functional alterations in the brain somatosensory system. Brain structural and functional measures and behavioral measures will be collected in 3 study groups: (1) neuropathic CLBP patients (2) non-neuropathic CLBP patients and (3) matched healthy control participants. CLBP patients will be classified as neuropathic or non-neuropathic in a two-step process based on a validated questionnaire (PainDETECT) and a standardized evaluation of pain tool, which combines sensory testing and validated questions targeting neuropathic pain symptoms. All participants will undergo functional brain imaging to collect resting state brain activity and structural brain images. Sub-cortical brain volumes, cortical thickness, white matter connectivity and functional connectivity will be compared among the three groups. In Aim 1, we will use fMRI to study brain functional and structural differences between neuropathic CLBP, non-neuropathic CLBP and healthy matched controls. In Aim 2, we will explore sex differences in brain structure and function in neuropathic CLBP patients, compared to non-neuropathic CLBP and healthy controls. Our long-term goal is to use the brain biomarkers of neuropathic CLBP derived from this proposal for future validation studies across sites, and for developing novel therapeutic targets both in humans and in animal models.